The Lower Jurassic Navajo Sandstone, along with the equivalent Nugget and Aztec sandstones represents the largest sand dune desert (called an “erg”) in the history of this planet. This erg existed for about 10 million years. Body fossils of animals that lived in this desert are rare, but trace fossils like tracks are fairly common. Studies in recent decades have described fossil burrows preserved in Navajo Sandstone sites near Moab. These burrows were likely created by Tritylodondids, a lineage of animals related to the mammals, whose body fossils and likely tracks are found in the Navajo Sandstone and underlying Kayenta Formation.

A variety of trace fossils are associated with small areas of carbonate rock that were created by interdunal lakes where groundwater created oasis pockets. The dunes and mounds near Moab are present in aeolian (wind-blown dune) sandstone beds, sometimes below lake-formed carbonate layers. Determining the origin of the branching structures isn’t easy because many similar structures include rhizoliths (fossil root casts), fluid escape features (liquefaction), and others. Two different types of structures have been attributed to burrowing animals in studies by paleontologists specializing in ichnology – the study of trace fossils.

“Type I” burrow structures have dense and complex networks of interconnected Y-shaped and T-shaped branching tunnels. These burrows also include inclined ramps that tie into low mounds. The walls of the tunnels are mostly smooth and the structures have similarities with burrows created by social animals (a modern example would be prairie dogs). A few specimens described by Riese and others (2011) show striations on the burrow walls that were likely created by the animal’s claws as it dug the tunnels.